Studies of the effectiveness of transport sector interventions in low‐ and middle‐income countries: An evidence and gap map

Abstract Background There are great disparities in the quantity and quality of infrastructure. European countries such as Denmark, Germany, Switzerland, and the UK have close to 200 km of road per 100 km2, and the Netherlands over 300 km per 100 km2. By contrast, Kenya and Indonesia have <30, Laos and Morocco <20, Tanzania and Bolivia <10, and Mauritania only 1 km per 100 km2. As these figures show, there is a significant backlog of transport infrastructure investment in both rural and urban areas, especially in sub‐Saharan Africa. This situation is often exacerbated by weak governance and an inadequate regulatory framework with poor enforcement which lead to high costs and defective construction. The wellbeing of many poor people is constrained by lack of transport, which is called “transport poverty”. Lucas et al. suggest that up to 90% of the world's population are transport poor when defined as meeting at least one of the following criteria: (1) lack of available suitable transport, (2) lack of transport to necessary destinations, (3) cost of necessary transport puts household below the income poverty line, (4) excessive travel time, or (5) unsafe or unhealthy travel conditions. Objectives The aim of this evidence and gap map (EGM) is to identify, map, and describe existing evidence from studies reporting the quantitative effects of transport sector interventions related to all means of transport (roads, rail, trams and monorail, ports, shipping, and inland waterways, and air transport). Methods The intervention framework of this EGM reframes Berg et al's three categories (infrastructure, prices, and regulations) broadly as infrastructure, incentives, and institutions as subcategories for each intervention category which are each mode of transport (road, rail trams and monorail, ports, shipping, and inlands waterways, and air transport). This EGM identifies the area where intervention studies have been conducted as well as the current gaps in the evidence base. This EGM includes ongoing and completed impact evaluations and systematic reviews (SRs) of the effectiveness of transport sector interventions. This is a map of effectiveness studies (impact evaluations). The impact evaluations include experimental designs, nonexperimental designs, and regression designs. We have not included the before versus after studies and qualitative studies in this map. The search strategies included both academic and grey literature search on organisational websites, bibliographic searches and hand search of journals. An EGM is a table or matrix which provides a visual presentation of the evidence in a particular sector or a subsector. The map is presented as a matrix in which rows are intervention categories (e.g., roads) and subcategories (e.g., infrastructure) and the column outcome domains (e.g., environment) and subcategories as (e.g., air quality). Each cell contains studies of the corresponding intervention for the relevant outcome, with links to the available studies. Included studies were coded according to the intervention and outcomes assessed and additional filters as region, population, and study design. Critical appraisal of included SR was done using A Measurement Tool to Assess Systematic Reviews (AMSTAR ‐2) rating scale. Selection Criteria The search included both academic and grey literature available online. We included impact evaluations and SRs that assessed the effectiveness of transport sector interventions in low‐ and middle‐income countries. Results This EGM on the transport sector includes 466 studies from low‐ and middle‐income countries, of which 34 are SRs and 432 impact evaluations. There are many studies of the effects of roads intervention in all three subcategories—infrastructure, incentives, and institutions, with the most studies in the infrastructure subcategories. There are no or fewer studies on the interventions category ports, shipping, and waterways and for civil aviation (Air Transport). In the outcomes, the evidence is most concentrated on transport infrastructure, services, and use, with the greatest concentration of evidence on transport time and cost (193 studies) and transport modality (160 studies). There is also a concentration of evidence on economic development and health and education outcomes. There are 139 studies on economic development, 90 studies on household income and poverty, and 101 studies on health outcomes. The major gaps in evidence are from all sectors except roads in the intervention. And there is a lack of evidence on outcome categories such as cultural heritage and cultural diversity and very little evidence on displacement (three studies), noise pollution (four studies), and transport equity (2). There is a moderate amount of evidence on infrastructure quantity (32 studies), location, land use and prices (49 studies), market access (29 studies), access to education facilities (23 studies), air quality (50 studies), and cost analysis including ex post CBA (21 studies). The evidence is mostly from East Asia and the Pacific Region (223 studies (40%), then the evidence is from the sub‐Saharan Africa (108 studies), South Asia (96 studies), Latin America & Caribbean (79 studies). The least evidence is from Middle East & North Africa (30 studies) and Europe & Central Asia (20 studies). The most used study design is other regression design in all regions, with largest number from East Asia and Pacific (274). There is total 33 completed SRs identified and one ongoing, around 85% of the SR are rated low confidence, and 12% rated as medium confidence. Only one review was rated as high confidence. This EGM contains the available evidence in English. Conclusion This map shows the available evidence and gaps on the effectiveness of transport sector intervention in low‐ and middle‐income countries. The evidence is highly concentrated on the outcome of transport infrastructure (especially roads), service, and use (351 studies). It is also concentrated in a specific region—East Asia and Pacific (223 studies)—and more urban populations (261 studies). Sectors with great development potential, such as waterways, are under‐examined reflecting also under‐investment. The available evidence can guide the policymakers, and government‐related to transport sector intervention and its effects on many outcomes across sectors. There is a need to conduct experimental studies and quality SRs in this area. Environment, gender equity, culture, and education in low‐ and middle‐income countries are under‐researched areas in the transport sector.

rated low confidence, and 12% rated as medium confidence. Only one review was rated as high confidence. This EGM contains the available evidence in English.
Conclusion: This map shows the available evidence and gaps on the effectiveness of transport sector intervention in low-and middle-income countries. The evidence is highly concentrated on the outcome of transport infrastructure (especially roads), service, and use (351 studies). It is also concentrated in a specific region-East Asia and Pacific (223 studies)-and more urban populations (261 studies). Sectors with great development potential, such as waterways, are under-examined reflecting also under-investment.
The available evidence can guide the policymakers, and government-related to transport sector intervention and its effects on many outcomes across sectors.
There is a need to conduct experimental studies and quality SRs in this area. Environment, gender equity, culture, and education in low-and middle-income countries are under-researched areas in the transport sector.

Plain language summary
The evidence base for the impact of transport is unevenly distributed and under-reviewed This evidence and gap map (EGM) shows the available evidence, and gaps in the evidence, on the effectiveness of transport sector intervention in low-and middle-income countries.
The evidence is highly concentrated on the outcome of transport infrastructure (especially roads), service and use. It is also concentrated in a specific region, East Asia and the Pacific, and urban populations.
Sectors with great development potential, such as waterways, are under-examined, reflecting under-investment.

What is this map about?
Transport interventions can play a key role in the achievement of many of the United Nations sustainable development goals (SDGs). This EGM contains the evidence base for all forms of transport: roads, bridges and paths, railways and trams, sea, ports and inland waterways, and civil aviation. For each part of the transport sector, the interventions are divided into infrastructure, incentives and institutions.

What studies are included?
Eligible studies had to be studies of the effects of a transport intervention, which is either the transport infrastructure or service itself, or transport-related incentives or institutions.
Studies had to be impact evaluations designed to determine effects, including regression analysis. Before versus after, ex-ante studies, and modelling studies without an empirical application are not included.
The EGM contains 466 studies, of which 34 are systematic reviews (SRs).
What is the aim of this EGM?
The aim of this EGM is to identify, map and describe existing evidence from studies reporting the quantitative effects of transport sector interventions related to all means of transport: roads, rail, trams and monorail, ports, shipping and inland waterways, and air transport.
What are the main findings of this EGM?
The studies are concentrated by sector and by outcome. The majority of the studies are in the intervention category of roads, bridges and paths, being mainly about roads. Of the three subcategoriesinfrastructure, incentives, and institutions-infrastructure is the most studied.
There is a moderate number of studies on railways, but the large majority of these are from East Asia, notably China. There are few studies on the other two intervention categories: sea and inland waterways, and air.
The studies follow the infrastructure. The large number of Chinese rail studies reflects the rapid growth in the Chinese railway system. The lack of studies of inland waterways in Africa reflects the lack of investment in this mode of transport.
The most frequently reported outcomes relate to transport use, such as mode of transport and travel time. This is followed by health and education, and economic development outcomes. Other outcomes are environment, equity and culture.
There are very few studies of known adverse effects like displacement. Guasch et al., 2014). Transport costs are high in sub-Saharan Africa even when the road infrastructure is adequate, due to a of lack of competition. Such considerations are an important part of the overall policy framework (Hine & Starkey, 2014), but beyond the scope of this map, which is concerned with quantitative studies of effectiveness, that is, the difference transport makes to outcomes of interest.
It is thus argued that better transport is a key component to achieving several sustainable development goals (SDGs): "There are a number of SDG targets directly linked to transport, including SDG 3 on health (increased road safety), SDG 7 on energy, SDG 8 on decent work and economic growth, SDG 9 on resilient infrastructure, SDG 11 on sustainable cities (access to transport and expanded public transport), SDG 12 on sustainable consumption and production (ending fossil fuel subsidies) and SDG 14 on oceans, seas and marine resources. In addition, sustainable transport will enable the implementation of nearly all the SDGs through inter-linkage impacts.
Access to sustainable transport for all should be at the forefront, including for vulnerable groups such as women, children, persons with disabilities and the elderly". 4 However, the presence and extent of these benefits depend on context: there is a great difference between those living in remote rural areas with little contact with the outside world and residents of a slum next to a highway in a rapidly growing city. How they interact with and can benefit from transport policies, of course, varies greatly. The impact of transport also depends on factors, such as employment opportunities, access to markets and distribution of health and education facilities, and other factors that may affect the use of all of these. The map must capture this full range of relevant interventions and possible policies, as well as the possible harms which may arise from transport.
Time frame: The time frame within which effects are realised varies.
The adverse effects of displacement can happen almost immediately.
Access to services can be relatively fast, though may depend on the development of transport services. Market opportunities and so growth may take longer to be realised, as will possible cultural effects.
Possible adverse consequences of infrastructure investments: Transport can bring disadvantages to some: displacement to make way for construction, poor road safety, higher land prices, spreading air pollution and disease, reduced accessibility on foot, moving access to jobs and goods further away, and adverse cultural effects.
Whilst transport infrastructure and services generally improve access to social services, they may have adverse effects on both health and education through the role of transport in spreading disease (the Black Death, HIV/AIDS in Africa in the 1980s and 1990s, and COVID-19 in 2020-see, e.g., Apostolopoulos & Sonmez, 2006), accidents, and a busy road through a village stopping parents sending young children to school (Jeyaranjan et al., 2010). Over 80% of road traffic deaths are in developing countries (WHO, 2018).
Some of these factors are not captured in most analyses, so there is a risk that, if adverse effects are not measured, then the cost-effectiveness of transport investments is overstated and they may not produce the full range of expected benefits, hence the importance of the regulatory framework. Understanding how transport policies can produce growthinducing effects and have social benefits, whilst considering possible adverse effects can guide setting priorities in the strategic use of scarce resources, and setting the regulatory framework for, transport investments. The challenge for transport development is thus to realise the benefits whilst minimising the adverse consequences.

| The intervention
The intervention is the transport system itself and any intervention aiming to construct, improve, maintain or affect the use of that transport. The main categories in the map are modes of transport: roads, railways, trams and monorail, ports, shipping and inland waterways, and air transport. For each mode of transport, the subcategories are infrastructure, incentives, and institutions (including regulations). A single study may cut across the various subcategories for a specific form of infrastructure, or less commonly analyse multiple forms of transport (or the connections between them).
The subcategories are the three policies that contribute to improving transport networks: (i) infrastructure investments, (ii) price instruments (which we label more broadly as incentives), and (iii) regulations (Berg et al., 2016). More specifically: • Infrastructure entails building new transport infrastructure (e.g., roads, railways, ports, or airports), upgrading existing links and technology, or improving transport services.
• Incentives include subsidies or taxes to influence mode choice and transport behaviour (e.g., student fare reductions, tolls, parking fares, fuel taxes, and clean transport subsidies).
• Institutions (regulations) include rules to directly reduce emissions, such as fuel emission standards or driving restrictions or to organise the transport sector (e.g., freight, taxis, or buses) or standards for the construction of infrastructure.
Some policy interventions may affect supply, such as infrastructure, whereas others target demand, such as subsidies for transport.

| Why it is important to develop this EGM
Although there is no separate SDG for transport, of the 17 SDGs, seven goals (SDG 2,3,7,9,11,12 and 13) include one or more targets that address transport, both rural and urban; and four (SDG 2, 3, 9, and 11) make specific reference to transport and infrastructure (United Nations 2016). According to the Institute of Transportation and Development Policy, "this elevation of transport in SDGs recognises it as a key tool in reducing emissions, improving equity, and reducing poverty". Analysis of these goals identifies the following key aspects of transport in the SDGs: access (urban, rural, affordable for all), road safety, fuel type/efficiency; quality, reliability, resilient, and sustainable infrastructure; regional and trans-border transport; 4 https://sustainabledevelopment.un.org/index.php?page=view&type=20000&nr=802& menu=2993 MALHOTRA ET AL. | 5 of 38 sustainable urban transport for all; reduce vehicle emissions/air pollution in cities; reform fossil-fuel subsidies; rural/urban logistics, supply chain efficiency; and mitigation and adaptation of climate change.
The literature on the impact of transport policies covers a variety of interventions and outcomes at different levels, such as micro, meso and macro. Due to the wide variety of interventions, mechanisms and outcomes, a simple way to formalise the impact of transport policies is to how these policies affect the welfare of individuals or groups, improve regulation and infrastructure, would be quite useful. At the same time, as explained above, the expansion of transport in LMICs has brought out both positive and negative effects.
The literature on the impact of transport policies covers a variety of interventions and outcomes at different levels, such as micro, meso and macro. Due to the wide variety of interventions, mechanisms and outcomes, a simple way to formalise the impact of transport policies is to how these policies affect the welfare of individuals or groups, improve regulation and infrastructure, would be quite useful. At the same time, as explained above, the expansion of transport in LMICs may have both positive and negative effects.
The purpose of this map is to document all relevant studies, from all sectors, which analyse the effects of transport interventions. The nearest studies to what we will do is the ADB review of transport impact evaluations by Raitzer et al. (2019) and the review of transport corridors by Roberts et al. (2019). However, the first of these review was not systematic and more restricted to analysis by economists, and the second is restricted to large infrastructure. We have a broader disciplinary coverage than the first and broader topic coverage than the secod.

| Existing EGMs and/or relevant SRs
A map of evidence maps conducted in low-and middle-income countries identified no EGM conducted around transportation (Phillips et al., 2017). The lack of such a map was the rationale for undertaking this map. There is an on-going global map of road safety (Mohan et al., 2020). Table 1 lists some reviews of transport sector interventions.
These are illustrative of the sort of topics, which may be covered; they have not been screened to determine whether they include primary studies from low and middle-income countries.

| Objectives
The EGM aims to identify, map and describe existing evidence on the effects of transport sector interventions related to all means of transport (roads, railways, trams and monorail, ports, shipping and inland waterways, and air transport) in low-and middle-income countries. For each sector, these interventions are classified as infrastructure, incentives, and the institutional framework (including regulations). The primary outcomes of this EGM include transport infrastructure, economic impact, health and education, environmental, economic, and equity outcomes.

| Defining EGMs
While SRs aim to identify, assess and summarise research findings from studies on a (narrow) research question, the objective of EGMs is to provide a picture of the coverage of existing research literature on a given topic. As such, EGMs have a broader scope than SRs, and SRs go further than EGMs in processing the contents of the identified research. Another important difference between EGMs and SRs is how they are disseminated. SRs are disseminated as research reports or journal articles, where the answer to the research question is the key issue for readers. EGMs can also be disseminated as a report or an article, but the more user-friendly EGMs display its results in an interactive matrix. Identified studies are plotted in the matrix, so that the user can find evidence, or lack thereof, for his or her particular topic of interest, at a glance. EGMs are global public goods that attempt to democratise high quality research evidence for policy makers, practitioners, and public and research funders.
The EGM presented here includes evidence from impact evaluations and SRs. Any single study may appear in multiple cells if it covers more than one category or subcategory for either intervention or outcome. Type of population (as applicable) The target population for this EGM is populations living in low-and middle-income countries. Rural/urban and global regions by World Bank classification are included as population subgroups. These subgroups are added to the map as filters.

Types of interventions/problem
The EGM includes intervention categories, which are each mode of transport such as roads, paths, cycle lanes, bridges, railways, ports, shipping and inland waterways, and air transport, and the subcategories are infrastructure, information and incentives, and institutions. Table 2 shows the resulting set of intervention categories.
Since the subcategory labels are the same across all categories it is possible in the visual EGM to swap the categories and subcategories. The authors will present the map in both layouts.

Types of outcome measures
The outcomes are listed in outcome domains (see Table 3). Each domain has a number of subdomains. The map covers positive and adverse outcomes, with outcomes being broadly defined so as to capture unintended outcomes. The selection of outcomes is informed by the theory of change which is presented below ( Figure 3).

Types of study designs
There are many policy-relevant areas of research on transport, including barriers to access, costs and governance arrangements.
Qualitative data and studies can play an important role on complementing impact evaluations; see White (2011)  is at the heart of discussions on transport and development. In the Handbook of Transport and Development (in which the cases are mostly from developed countries), the authors state in the introduction that "Often it seems that development follows the transport infrastructure… But the causality is rarely in one direction and often the development form helps shape the transport infrastructure investments" (Hickman et al., 2015: 3).

This EGM includes ongoing and completed impact evaluations and
SRs of the effectiveness of transport sector interventions. This is a map of effectiveness studies. The impact evaluations include: • Experimental designs: RCTs and natural experiments.
• Non-experimental designs: (i) quasi-experimental designs using statistical methods to create a comparison group such as propensity score matching and regression discontinuity, (ii) regression-based designs such as instrumental variables and Heckman sample selection models; and (iii) other studies with a comparison group. Before versus after studies with no comparison group are not included.
• Regression designs which control for confounding variables.
We do not include before versus after studies, ex ante impact estimates including cost-benefit analysis, or modelling studies without an empirical application.

Types of settings (as applicable)
All included impact evaluations must have been conducted in low-and middle-income countries (LMICs) as defined by the World Bank. SRs containing evidence only from high-income countries are excluded.

Status of studies
We searched for and included completed and on-going studies.
We have restricted our search to English language. We did not exclude any studies-based on publication status or publication date.

Search methods and sources
The search strategy included 12 databases, and more than 20 relevant websites, and a hand search of more than 20 journals (see Figure 2). The authors also included grey literature from Google as well as the listed websites. We conducted bibliographic back-referencing of reference lists of all included SRs to identify additional primary studies and SRs.
In addition, we identified the developing country studies from the ongoing map of road safety interventions (Mohan et al., 2020). All screening was done independently by two people (SM, NDC) with a third-party arbitrator in case of disagreement (HW).

EGM Protocol
The EGM protocol was published on 21, January 2021 .

| Stakeholder engagement
The choice of transport as a map was based on the map of maps (Phillips et al., 2017) which identified a gap in this area, and was seen as a priority by the funder, that is the UK Foreign, Commonwealth, and Development Office (FCDO). Horizontal equity, also called fairness and egalitarianism, is concerned with the distribution of impacts between individuals and groups considered equal in ability and need; vertical equity is concerned with the distribution of impacts between individuals and groups that differ in abilities and needs, for example by income or social class (also called social justice, environmental justice and social inclusion) or in transportation ability and need otherwise known as universal design (Litman, 2018) We have engaged stakeholders in developing the evidence matrix at the various organisation that works on transport sector interventions. These include: • TERI University (Department of Civil Engineering) • IIT-Delhi, • and Independent Council for Road Safety International (ICORSI).

| Scope
The scope of this map covers (1)  • Health: Health is separated as there are many channels through which transport can affect health, both positive (access to health services, higher income, availability of more diversified diet, etc.) and negative (road traffic injuries, air pollution, and spreading disease). This framework is used to define the categories of interventions and the outcomes along the causal chain to be shown in the map.

| Description of population/geographic location/outcome categories
Included studies were those that include population from low-and middle-income countries and reported the transport sector intervention and on the main six outcomes.
The six main outcomes, which follow from the theory of change, are: • Transport infrastructure, services, and use • Economic Impact • Health and education • Culture

F I G U R E 3 General theory of change for transport interventions
• Environment • Economic and equity analysis.
The outcomes categories and subcategories are given in Table 2.

| Unit of analyses
In the EGM, where multiple papers exist on the same study only one is included if they are the same (e.g., working paper and a published version), the most recent open access version included in the EGM.
Where different papers from the same study report different outcomes then all such papers will be included.

Presentation
This EGM has two primary dimensions: intervention as rows and outcomes as columns.

| Data extraction and management
For impact evaluation and SRs, we used a standardised data extraction form (Annexure 1) to extract descriptive data from all the studies that met our inclusion criteria. Data extraction from each study included context/geographical information, population, study design and method, intervention types and outcomes type, and subcategory. Two researchers (S. M. and N. D. C.) conducted the data extraction for each study. Both coders were trained on the tool before starting. Disagreements were resolved through discussion with a third reviewer consulted as needed (H. W.).

| Tools for assessing the study quality of included reviews
All SRs were appraised for quality using the AMSTAR2 tool. Critical appraisal assessment was completed by two reviewers (S. M. and N. D. C.).
The 16 items in AMSTAR2 cover: Seven domains can critically affect the validity of a review and its conclusions (critical items 2, 4, 7, 9, 11, 13, and 15). The study's overall confidence ratings of the quality are high if there is no more than one noncritical weakness, medium if there is no critical weakness but more than one noncritical weakness, and low if there are one or more critical weaknesses.
We did not critically appraise the quality of the included impact evaluations but collected data on study design.  (Mohan et al., 2020). The majority of these studies (213) were excluded on closer examination at the coding stage, resulting in 170 additional studies on the map (see Figure 4). We also added 24 additional studies from the search to High Volume Transport Applied Research Programme (HVT) website.
As a result of both phases, we have included 466 studies that met our inclusion criteria: 432 impact evaluations and 34 SRs ( Figure 5).

| Overview-Interventions and outcomes
In the aggregate map (Table 3a) we present the aggregate map by intervention category (mode of transport) and outcomes. The most striking finding is that the dominance of studies on roads, bridges, and paths (which are mainly roads). There is a reasonable number of studies on rails and trams (which, as shown below, are mostly from East Asia, mainly China). There are very few studies indeed on transport by air, sea, and inland waterways, reflecting in part the neglect of these as a transport system.

Waterways remain an underused means of transport in Africa and
South Asia. The Ganges saw little growth in freight traffic from 1945 to 95, with modest growth thereafter. Freight on the Congo remained stagnant from 1945 to 2015. By contrast, traffic on the Yangtze increased more than fourfold over the same period . Of course, there is undoubtedly an endogeneity: economic development increases freight transport as well as better waterways facilitating economic development. Given the exogenous placement of waterways, analyzing their impact is a tractable problem, but there is no paper presenting this analysis. The closest study to this issue is that of  showing that access to ports substantially increases exports of cash crops such as coffee, tea, tobacco, and cotton in African countries.
The most commonly reported outcomes are access to transport, economic impact, and health and education in that order. Hence the most heavily evidenced cells-with over 100 studies per cell-are the three cells for these outcomes for roads, bridges and paths. Much of the map has few, or even no, entries, pointing to substantial evidence gaps. As already noted, these affect transport modes (air and water) and some outcomes, notably culture, but also economic and equity outcomes, environment and, health and education for all transport modes other than roads.
The distribution of the SRs follows roughly the same pattern as primary studies. Most reviews are about roads, with few on other means of transport. However, the main outcome is health-which is common for reviews as this is best established in health reviews.
However, there are also a reasonable number of reviews on economic development. A useful follow-on product from this map would be an overview of reviews contained in the map, one output of which would be to identify (in conjunction with additional analysis as to what is covered by the primary studies in the map) a list of potential topics for additional SRs. Table 3b shows the aggregate map with the categories now shown as infrastructure, incentives, and institutions. This shows that the most well-evidenced area is infrastructure across the main outcome categories already noted. There are a reasonable number of studies on institutions, mostly about transport use. There are the fewest on incentives, and again mostly on transport use.
In all these tables the totals do not sum since a study may appear in more than one cell.  . Under the 14th Five Year Plan (2021-2025) an additional 10,000 km of rail with be built. 5

| Evidence base by outcome category and subcategory
The evidence base is largest for the outcomes related to transport time or cost and transport modality (Figure 9). There are also many studies reporting economic development which includes growth, firm and enterprise development, and agricultural production. Other wellstudied economic outcomes are household income and poverty, employment, and migration. Other outcomes with reasonable evidence are health outcomes, air quality, and road safety. As already noted, there are few studies of cultural effects and on the adverse outcomes of displacement and habitat loss. SRs make up just 7% (34 out of 466) studies in the map. This is a low percentage compared to most other maps. For example, the disability map has 59 reviews out of a total of 166 studies, that is 36% . Transport is thus an under-reviewed area. As proposed above, the map should be used to identify additional SRs which would be of interest to decision-makers.
Region-wise study design: In East Asia and Pacific the most used study designs are other-as for the whole map-is other regression design, followed by nonexperimental designs with a comparison group. The same pattern is seen in other regions except for Latin America and Caribbean, where comparison group designs are most common ( Figure 12).

Population groups
The effects of transport were studied for the urban population in a just over half of the studies (56% of total 466 studies), compared to under quarter (21%) considering rural population, and 19% covering both rural and urban. Urban studies look disproportionately at transport use, whereas rural studies are more concerned with economic impact ( Figure 13).

| Status of included studies
There are 460 completed studies and 6 ongoing studies included in the map.

| Quality appraisal in included reviews
There are 34 SRs. We critically appraised the quality of the reviews by using AMSTAR-2. Among the included reviews, 85% of the SRs rated as low and 12% rated as medium confidence in study findings.
Only one review was rated as high confidence.
As shown in Figure 14, major limitations were the absence of risk of bias analysis, not undertaking meta-analysis-only 1 study executed a meta-analysis, failure to use two screeners and coders (or at least a failure to report doing so), failure to have a protocol, and not declaring sources of funding. This assessment of the shortcomings in existing reviews reinforces the case for commissioning a new programme of reviews of transport studies.  (Figure 15). The East Asia and Pacific region accounts for the largest share of studies (40%), with most of these coming from China. Just over half of studies concern the urban population (56%).
Sectors other than roads are relatively neglected in the evidence base. Whilst there are a sizeable number of studies on railways, most of these are from one country (China). There is very little evidence on waterways, whose potential remains unrealised especially in sub-Saharan Africa.
There is very little evidence on equity analysis and culture. It is possible that cultural effects are more studied in qualitative literature.
Only six studies among 466 about culture outcomes and only four studies measured noise pollution. Only 18 studies measured the effects of transport on gender equity, 21 studies applying ex-post cost-benefit analysis, and two studies reported findings for transport equity.
The low ratio of reviews to primary studies makes this an underreviewed area. Moreover, most of the reviews have methodological shortcomings, such as a failure to conduct and use the risk of bias analysis and to undertake meta-analysis where appropriate.

| Areas of major gaps in the evidence
There are many blank cells in the intervention categories in civil We found notable gaps in the evidence related to the intervention on the ports, shipping and no evidence on waterways. The evidence is very much concentrated on infrastructure development and use, economic impact, and health outcomes. There is a striking gap in studies that focus on the effects of transport on cultural heritage and diversity.
There is a lack of evidence on outcomes such as noise pollution, values, language, and social cohesion, transport equity, and displacement There is very little evidence on environmental outcomes such as air quality. There are also few studies on equity issues such as gender equity.
The extent to which these gaps need to be filled depends on whether these are priority areas for policy-makers.

| Potential biases in the mapping process
In terms of biases, in the selection process, we have selected evidence available in the English language.

| Limitations of the EGM
i. Eligible studies were restricted to those published in English.
ii. Searching the grey literature is challenging, and consequently, some eligible studies may have been missed.
F I G U R E 14 AMSTAR2 assessment F I G U R E 15 Evidence concentration and gaps in categories among outcome 6.5 | Stakeholder engagement throughout the EGM process We have engaged stakeholders on the evidence matrix at the various organisation that work on transport sector interventions. These include TERI University, Department of Civil Engineering, IIT-Delhi, and Independent Council for Road Safety International (ICORSI).
The draft report was shared with ADB, and African Development Bank as well FCDO (DFID) and MCC.

| AUTHORS' CONCLUSIONS
The mapping exercise has two goals: • Facilitate access to, and use of, research on the effectiveness of transport interventions through the online interactive visualisation of the map and accompanying list of references; and • Identify priority areas for SRs and impact evaluations for transport.

| Implications for research, practice, and/or policy
The map points to several gaps in the evidence base with respect to primary studies. It also points to the lack of reviews and the methodological shortcomings in most existing reviews. Some of the implications for further research are: • Efforts needed so that the key funders and researchers in the transport field reach the consensus to identify the priority area for research with weak evidence synthesis.
• Future research should study the interventions related to incentives and institutions and regulations in railways, port, shipping and waterways, and civil aviation.
• To fill the important gaps in this sector, there is a need for more studies on the areas of environment, education, culture, gender equity, and transport equity.
• The geographical base of evidence needs to be expanded, the majority of the studies to date are from East Asia and Pacific.

CONTRIBUTIONS OF AUTHORS
The lead author is the person who develops and co-ordinates the EGM team, discusses and assigns roles for individual members of the team liaises with the editorial base and takes responsibility for the ongoing updates of the EGM.

Content expertise:
Nina Blöndal has conducted several impact evaluations of transport interventions and authored a chapter on transport impact evaluation for the ADB Guidebook. Dr. Howard White co-edited a special issue of the Journal of Development Effectiveness on infrastructure impact evaluations including contributing a paper on mixed methods in infrastructure studies.
SR method expertise: All authors are experienced systematic reviewers, which means that they are proficient in conducting various processes in an EGM, such as screening, quality assessment and coding. Howard White will provide technical support for the conducting the review.
EGM methods expertise: Howard White as CEO provides technical and strategic support for the development of the EGM.
All team members have previous experience in SR methodology, including search, data collection, statistical analysis, theory-based synthesis, which mean they are proficient in carrying out the various processes in an EGM, such as search, eligibility screening, quality assessment and coding.
Information retrieval expertise: John Eyers is a trained information retrieval specialist and has experience of supporting over 50 systematic maps and reviews in social sciences areas.

DECLARATIONS OF INTEREST
Howard White is the CEO of the Campbell Collaboration. He has no role in the editorial process for this EGM.

PLANS FOR UPDATING THE EGM
We plan to update the map (or support others in doing so) when sufficient further studies and resources become available.

DIFFERENCES BETWEEN PROTOCOL AND MAP
None.

SOURCES OF SUPPORT
This EGM is supported by the UK Foreign, Commonwealth and Development Office (FCDO) under its support for the Centre for Excellence for Development Impact and Learning (CEDIL).